Rod lenses are optical elements and devices that are formed from an elongated element that is transparent and in particular made from quartz glass. The element has the following typical geometric configuration: edge length (L)×height (H)×thickness (D), wherein the light entry and exits surfaces are defined by L×H and wherein the edge length (L) is many times greater than the height (H). When the edge dimensions (D) and (H) form the two legs of a right triangle, this yields the hypotenuse which simultaneously forms the diameter of the rod lens face. Depending on the application the light entry- and exit surfaces are formed as planar, convex or concave functional surfaces through classic optical processing in L-direction and also in H-direction. Rod lenses are used in display techniques, LED light source arrangements and optical imaging devices.
For producing a rod lens initially a rod lens base element is produced e.g. as a square rod which subsequently obtains the final shape of the rod lens through grinding and polishing processes. The rod lens element made from synthetic quartz glass or the rod lens produced therefrom has to have a high degree of optical homogeneity over its entire length. In particular they have to be free from bubbles, enclosures, layers and cords. Furthermore the quartz glass should have an OH content ≧1,000 ppm and a H2 content ≧0.8×1018 mol/cm3.
In order to achieve these high optical requirements typically quartz glass from multi stage melting processes is being used. This means a prefabricated quartz glass element is finished through homogenization steps (DE 10 2005 043 289 B3) and/or additional loading of H2 (DE 696 13 268 T3) in a complex manner. The quartz glass cylinders thus created have to be thermally formed in an additional process. This means the glass material is put into a mold that is made for example from graphite and subsequently brought to a softening- or flow temperature in a sinking kiln. The glass material thus sinks into the mold. For flat molds a glass surface (sinking block) is provided which is subsequently separated into particular rod lens elements. Due to the extreme rod lens geometry (L many times greater than H) multiple sinkings are required until the eventually desired shape of the rod lens element is achieved.
The sinking method recited supra requires precise control of a position of inhomogeneities within the glass cylinders. It has to be assured that inhomogeneities provided in the quartz glass cylinder, in particular possible layers do not change their orientations in the sinking processes so that the predetermined propagation direction of the light remains oriented perpendicular to these layers in the finished rod lens and does not influence the light propagation direction (D) of the rod lens over its entire edge length.
Through optical components that were newly introduced in recent years, in particular organic LEDs, so called OLEDs, rather long rod lenses are required that are relatively narrow relative to their height. These rod lenses have an edge length of approximately 800 mm and more. Furthermore the rod lenses which are exposed to intense laser irradiation have to have a florescence within the lens element that is as small as possible. In conjunction therewith a high degree of transmissivity of the lens element is required in the ultraviolet spectral range for various applications.
It is apparent that rod lenses with the required properties can only be produced through a sinking method in a very complex manner and only within a long production time. Thus, in particular precise control of uniform optical properties over the great length of the rod lens element causes problems. Through the high temperatures during the sinking process of 1700° C. or more quite frequently undesirable changes occur in the material properties of the glass materials. The contact with the graphite mold also induces uncontrolled property changes of the glass. Longer rod lens elements furthermore can only be produced in a multi stage process and thus in a time consuming manner. Furthermore complex finishing has to be performed upon the last sinking block which partially has to be performed with considerable material loses. Furthermore, the graphite molds have a finite service life and their manufacture is expensive. Additionally, the quality of the glass material and the successful execution of the sinking method can eventually only be checked at the finished rod lens.